Known in the field of infrared detectors are devices arranged in array form and capable of operating at room temperature, that is to say devices not requiring cooling, unlike the detection devices called quantum detectors which themselves require operation at very low temperature, typically at liquid nitrogen temperature.
These uncooled detectors conventionally use the change in a physical parameter of a suitable material as a function of temperature in the region of 300 K (540° R). In the case of bolometric detectors, this physical quantity is the electrical resistivity.
Such an uncooled detector generally combines:
means for absorbing the infrared radiation and converting it into heat;
means for thermally insulating the detector in such a way as to allow it to heat up under the action of the infrared radiation;
thermometry means, which in the context of a bolometric detector employ a resistive element; and
means for reading the electrical signals delivered by the thermometry means.
Detectors intended for infrared imaging are produced in the form of an array of elementary detectors, in one or two dimensions, on a substrate, generally made of silicon, which includes means for electrically exciting the said elementary detectors and means for preprocessing the electrical signals generated by these elementary detectors.
These electrical excitation and preprocessing means are formed on the substrate and constitute a read circuit.
Monolithic integration of the detectors into the corresponding read circuit is advantageous from the standpoint of fabrication costs. However, it is also possible to hybridize a detector array on such a read circuit.
A device comprising an array of elementary detectors and an associated read circuit is generally placed in a case and connected, especially electrically, to the external environment by conventional techniques. In such a case, the pressure is reduced so as to limit the thermal losses. This case is furthermore provided with an infrared window transparent to the radiation to be detected.
To observe a scene by means of this detector, the scene is projected through a suitable optic onto the array of elementary detectors and the varying electrical stimuli are applied, via the read circuit (also provided for this purpose), to each of the elementary detectors or to each row of such detectors, so as to obtain an electrical signal constituting the image of the temperature reached by each elementary detector.
This signal undergoes relatively sophisticated processing by the read circuit, and then possibly by an electronic device external to the case, so as to generate a thermal image of the observed scene.
The performance of uncooled bolometric detectors depends essentially on:
controlling the production and integration of the most effective bolometric materials;
controlling the design and construction of the elementary detectors, in the form of microbridges, that is to say lightweight and delicate structures, thermally isolated from the read circuit;
the quality of application of these detectors and of the various correction functions that are used in the read circuit and in other ancillary devices; and
controlling the techniques for packaging them in a low-pressure sealed case.
The present invention is more particularly aimed at controlling the design and construction of the microbridges. The aim of the invention is in fact to allow very effective bolometric detectors to be obtained using techniques that are relatively simple to employ.
The prior art describes a number of different ways of arranging the various constituent elements of the elementary detectors.
A distinction may principally be made between coplanar-electrode detectors and parallel-electrode detectors (i.e. having a “sandwich” structure). Document US-A-5 021 663 describes, for example, a bolometric detector of the type in question.
Although the invention is more readily applicable in coplanar-electrode detectors, in which the electrical current passing through the structure in operation flows in the plane of the elementary detector, it is also applicable in parallel-electrode detectors.
Document FR-A-2 752 299 describes a bolometric detector comprising a read circuit and one or more elementary detectors which themselves have a sensitive part integrating in particular a layer of bolometric material and two conducting electrodes, and at least one support element for this sensitive part, and in which layer the electrodes are interdigitated. FIG. 1 shows schematically a representation of this elementary bolometric detector.
In this detector, the read circuit (1) is covered with a reflective metal layer (2) intended to reflect the infrared radiation not absorbed by the bolometer itself, and placed about 1.5 to 2.5 μm (0.00006 in.–0.00010 in.) above the said reflector (so as to optimize the detection around a wavelength of 10 μm (0.00039 in.), corresponding approximately to the useful detection range of these detectors).
Such positioning is performed by means of essentially vertical structures (3). These structures, which will be called “pillars” in the rest of the description, are themselves electrically conducting and thus make it possible to transfer the excitation potentials to the conducting parts or electrodes (5) of the bolometric detector via elongate plane structures (4), which are also electrically conducting but thermally resistive. These elongate plane structures will be called “arms” in the rest of the description.
The thermal resistance, also called “thermal isolation”, is intended to allow the bolometric material to heat up under the effect of the infrared radiation.
The volume between the body of the bolometer and the reflector is devoid of material away from the pillars (3), so as to prevent thermal losses through solid conduction. This volume is usually filled with a low-pressure gas so as to limit convection and conduction by the gas.
In this type of device, the read circuit applies, via the pillars (3) and the arms (4), and via at least two conducting parts or electrodes (5), an electrical current that passes through the structure parallel to the plane of the bolometric detector. This current flows through a bolometric material (6), the resistivity of which varies with temperature. The most widely used materials for this purpose are vanadium oxide and amorphous silicon.
The electrodes (5) are produced as a thin conducting layer, ordinarily made of metal. They also serve to absorb the infrared radiation. In FIG. 1A it may be seen that these electrodes are placed on the upper surface of the bolometric material (6). The internal surface suspended from the bolometer, that is to say away from the pillars (3) and arms (4) in FIG. 1A, almost always takes the form of virtually equipotential surfaces (5) and of resistive surfaces, the extent of which is defined by the spaces between the parts (5). In the rest of the description a distinction is made between the parts (6A) and the parts (6B) of the layer of bolometric material (6), depending on whether the excitation current passes through them (6A) or such a current does not pass through them (6B).
The performance of a bolometric detector is conventionally expressed by the NEdT (Noise Equivalent Differential Temperature).
It may be demonstrated, assuming that the bias level is sufficient for the electrical noise in the detector to be dominated by the low-frequency noise (Nlf), called “1/f noise”, which is a characteristic in particular of amorphous materials, that the noise equivalent differential temperature is given by the equation:
      1    NEdT    =                    k        ⁡                  (                      W            ·            L            ·            E                    )                            1        /        2              ·    TCR    ·          R      th      in which:
k is a proportionality coefficient that is not worth explaining in detail here, which includes the bandwidth of the read circuit, the 1/f low-frequency noise level of the bolometric material (6), the area of the elementary detector and the infrared absorption efficiency of the elementary detector;
W and L are the electrical width and length respectively of the current lines through the bolometric material (6), these being shown in FIGS. 1A and 2A;
E is the thickness of the bolometric material (6) over the area relating to the current lines, the said area being defined by the dimensions W and L;
TCR is the relative resistance variation near the operating temperature (dR/RdT), which is a characteristic of the bolometric material employed, where R is the electrical resistance seen between the two current injection poles (the two pillars (3) and T is the temperature of the microbridge; and
Rth is the thermal resistance between the central “solid” part of the bolometer, which heats up under the effect of the infrared radiation, and the read circuit (1), the temperature of which is constant or very slightly variable.
This assumption is justified by the fact that the noise equivalent differential temperature derives from the calculation of the S/N, where S is the signal delivered by the detector and N is the electrical noise.
The detection signal S, proportional to the electrical current “i” flowing through the bolometer(S=K1·i),must be a maximum, while the low-frequency noise is also proportional to “i”(Nlf=K2·i).
Thus, when the electrical current is increased to improve the signal, a moment arrives when the low-frequency noise (Nlf) dominates over the other sources of noise that are independent of the current value, the “white” (frequency-independent) noise of which is typically generated by the bolometric detector.
The signal-to-noise ratio is optimum when it approaches its limit value K1/K2 for a sufficient value of the excitation current, even for materials that naturally have a low noise at low frequency.
The bolometer is therefore typically dominated by the 1/f noise under the bias conditions that optimize the noise equivalent differential temperature NEdT.
These various quantitative aspects may be found in document FR-A-2 796 148.
It is apparent from this equation that the detection performance of the bolometric detectors is related to the volume of bolometric material through which the lines of excitation current pass, that is to say equal to the product of the area (W·L) of the resistive parts (6A) multiplied by the thickness E of the said bolometric material.
The parts (6B) of the bolometric material, that is to say those through which the excitation current does not pass, do not contribute to the definition of the detection performance of the bolometric detector, as practically no current passes through the said bolometric material in these regions, these being much more electrically resistive than the layers that define the parts (5).
In the case of a detection array, the repeat pitch of the elementary detectors in the two dimensions of the plane is defined by “p”.
To achieve optimum detection performance, all that is required, using the above analysis, is to arrange parts (6A) as a polygon of length L and width W, withL·W=p2 in order to optimize the detection performance.
This quantity p2 represents the upper bound of the parameter L·W, since, from the technical standpoint, it is necessary to reserve part of the area p2 for placing the spaces that separate the elementary detectors from one another, and at least the pillars (3), the arms (4) and the regions (6B), the area of which cannot be zero.
These quantities L and W in a typical arrangement have been shown in FIG. 1.
However, it is observed that the infrared radiation absorption rapidly decreases when the area of the electrodes (5) is reduced to the gain of the area of the surfaces (6A).
In practice, the best performance in terms of bolometric resolution is obtained when there is a balance between the area of the electrodes (5) and that of the regions (6A).
This means that about half of the internal area of the bolometer (that of the parts 6B) cannot be used to optimize the current lines, that is to say to maximize the total area L·W of the polygon(s) through which the current lines pass.
The expression for evaluating the bolometric resolution of NEdT also shows that the performance is improved if the thickness E of the layer of bolometric material (6) is increased as well as the dimensions of the quantities W and L.
However, such an increase in thickness correspondingly increases the thermal mass Cth of the bolometer, this thermal mass appearing in the definition of the thermal time constant through the equationτth=Cth·Rth,which also constitutes one of the important parameters from the standpoint of the use of the bolometric detector as it defines the maximum rate at which the detector in question can follow a temperature change at any point in the observed scene.
According to document FR-A-2 752 299, the total thermal mass of the bolometric detector is largely determined by the mass of the bolometric material (6), and any increase in thickness of this layer is accompanied by an almost proportional increase in the overall thermal mass.
Consequently, the gain in thermal resolution NEdT achieved by this greater thickness of bolometric material is compensated for by the increase in the thermal time constant τth. The overall optimization of the bolometer therefore assumes that the thermal time constant, and therefore the thickness of the layer of bolometric material (6), is adjusted to the maximum value compatible with the operating frequency envisaged by the user.
In other words, the thickness of the bolometric material is therefore not a free optimization parameter.
Thus, the bolometric detector according to document FR-A-2 752 299 cannot be improved in terms of noise equivalent differential temperature owing to the need to take into account the thermal time constant in order to allow effective use of such a bolometric detector.
To improve the noise equivalent differential temperature of a bolometric detector, the aforementioned document FR-A-2 796 148 proposes a configuration that relaxes the constraint associated with the quality of radiation absorption and of balancing the areas of the electrodes (5) and of the regions (6A) of the bolometric material.
According to the teachings of that document, the contact parts (6B) between the electrodes (5) and the bolometric material (6) are reduced to small areas of narrow elongate shape. Furthermore, the electrodes (5) are isolated from the bolometric material over essentially their entire surface by the insertion of an insulation layer (7).
This technique makes it possible to use most of the surfaces (6B) to optimize the polygon(s) of area W·L and thus obtain a substantial improvement in performance.
However, this result is obtained, on the one hand, by adding an additional mass, associated with the use of the insulating material (7), and stems, on the other hand, from a not insignificant increase in production complexity. It also has the drawback of resulting in a loss of performance owing to the pinching of the current lines around the ends of the parts (6B) in the case in which the electrodes (5) have an interdigitated configuration, as shown in FIG. 2A.
Now, this type of configuration is practically inevitable when a high-resistivity bolometric material, such as amorphous silicon and related materials, is used to obtain an electrical resistance R ranging from a few 105 ohms to 106 ohms, which is practical from the standpoint of the read circuit.
It should in fact be recalled that one of the difficulties that a person skilled in the art has to encounter when defining the structures of a bolometric detector is how to achieve an electrical resistance R around room temperature that is tailored to the options of the read circuit.
This is because not just any resistance value is necessarily practical to be employed with the type of read circuit that the designer of the system intends to use, and it is in general more practical, in order to optimize the functions of the read circuit, for this resistance R to be determined by constraints specific to this circuit, rather than by constraints generated by the bolometric detector as such. By neglecting the resistances constituted by the pillars (3) and the arms (4), this resistance is defined by the equation:R=ρ·L/(W·E)where ρ is the electrical resistivity of the bolometric material in the vicinity of the opening temperature.
The configuration of the electrodes (5) straight away defines the width W and length L and the interdigitation, as for example shown in FIGS. 1A and 2A, and offers a certain degree of freedom.
However, the possible variations in the interdigitation of the electrodes (5) are not in practice very many with respect to the areas p2 usually employed (less than 50·50 μm2 (0.00197·0.00197 in.2) and with a pitch (width+space) of electrode designs of more than about 8 μm, in order to avoid diffraction phenomena between 8 and 14 μm (0.00031 in. and 0.00055 in.) in wavelength, that is to say within the wavelength range corresponding to infrared imaging.
The configuration of the current lines in the case shown in FIG. 2A can be modelled by, on the one hand, the three rectangular regions bordered by the parts (6B) in their parallel straight portion and, on the other hand, the two regions (8) corresponding to the ends of the parts (6B) internal to the bolometric detector.
In the rectangular regions, the density of the current lines is uniform and local quantities Wr and Lr are immediately defined in which the subscript r refers to the rectangular regions.
These regions define an electrical resistance equal to Rs·L/W, where Rs is the sheet resistance(Rsρ/E)of the bolometric material.In contrast, in the regions (8), the density of the current lines is variable, namely markedly higher in the vicinity of the tips of the regions (6B) than in the interior of the rectangular regions, and markedly lower in the vicinity of the opposite part (6B).
If the two regions (8) are grouped together in the form of a disk of internal radius r1, taking r1 as being the radius of the tips of the two regions (6B), then the external radius of these regions is L+r1. In practice, r1 may be likened to the half-width of the elongate feature of the parts (6B) of FIG. 2A.
It is readily demonstrated that the electrical resistance of this disk is given by the equation:
                    R        s                    2        ⁢        π              ·    ln    ⁢                    L        +                  r          1                            r        1              .  The width Wt of the rectangle of length L equivalent to the regions (8), in which the subscript “t” relates to the tip regions, is given by the equation:
                    R        s            ⁢      L              W      t        =                                          R            s                                2            ⁢            π                          ·        ln            ⁢                        L          +                      r            1                                    r          1                    ⁢                          ⁢              i        .        e        .                                  ⁢                  W          t                      =          2      ⁢      π      ⁢                          ⁢              L        /        ln            ⁢                                    L            +                          r              1                                            r            1                          .            
According to the example shown in FIG. 2A, for a bolometer with an overall size of 35 μm (0.00138 in.), including the space between adjacent bolometric detectors with L=6.5 μm (0.00026 in.) and r1=0.5 μm (0.00002 in.) for example, this results in an equivalent electrical width Wt=15.5 μm (0.00061 in.) (for both regions (8)), substantially less than the length of the rectangular outline (in dotted lines in FIG. 2A) that would represent the maximum electrical width W that could be used near the regions (8) in order to optimize the produce W·L and therefore the noise equivalent differential temperature or thermal resolution of the bolometer. This length of the rectangular trace would in fact be about 28 μm (0.00110 in.) according to the scale of the realistic example shown in FIG. 2A. The effective total “electrical” widthW=Wt+Wt (a solid line along the central equipotential between the parts (6B)) would be close to 60 μm (0.00236 in.) for a maximum electrical width (shown by the dotted lines) of about 73 μm (0.00287 in.). This difference represents a loss of about 11% in performance compared with an ideal bolometric detector, which would not have localized tip effects.
To summarize, apart from the drawbacks already mentioned, the configuration proposed in that document could be improved as regards thermal resolution of the bolometric detector.
There has also been described, in document US-A-5 367 167, a bolometric detector comprising two coplanar electrodes located on the same face of the layer of bolometric material, and a conducting layer located on the other face of this layer of bolometric material. The function of this conducting layer is to absorb the infrared radiation that it is desired to detect and an electrically insulating layer must separate the conducting layer from the body of the detector. This therefore makes production very complex. Moreover, having to arrange the electrodes further apart restricts the application of this bolometric detector to low-resistivity bolometric materials, such as typically vanadium oxides.